Isotropic soap phase

Isotropic properties, silicon, 22 482, 483t Isotropic soap phase, 22 726 Isovaleraldehyde. See also 3-Methyl butanal... 

It would be incomplete for any discussion of soap crystal phase properties to ignore the colloidal aspects of soap and its impact. At room temperature, the soap—water phase diagram suggests that the soap crystals should be surrounded by an isotropic Hquid phase. The colloidal properties are defined by the size, geometry, and interconnectiviness of the soap crystals. Correlations between the coUoid stmcture of the soap bar and the performance of the product are somewhat quaUtative, as there is tittle hard data presented in the literature. However, it might be anticipated that smaller crystals would lead to a softer product. Furthermore, these smaller crystals might also be expected to dissolve more readily, leading to more lather. Translucent and transparent products rely on the formation of extremely small crystals to impart optical clarity. 

Binary Soap-Water System Mixtures of soap in water exhibit a rich variety of phase structures (4, 5). Phase diagrams chart the phase structures, or simply phases, as a function of temperature (on the y-axis) and concentration (on the x-axis). Figure 2.1 shows a typical soap-water binary phase diagram, in this case for sodium pahnitate-water. Sodium palmitate is a fully saturated, 16-carbon chain-length soap. At lower temperatures, soap crystals coexist with a dilute isotropic soap solution. Upon heating, the solubility of soap increases in water. As the temperature is increased the soap becomes soluble enough to form micelles this point is named the Krafft point. The temperature boundary at different soap concentrations above which micelles or hquid crystalline phases form is named the Krafft boundary (5). 

Ekwall and Baltcheffsky [265] have discussed the formation of cholesterol mesomorphous phases in the presence of protein-surfactant complexes. In some cases when cholesterol is added to these solutions a mesomorphous phase forms, e.g. in serum albumin-sodium dodecyl sulphate systems, but this does not occur in serum albumin-sodium taurocholate solutions [266]. Cholesterol solubility in bile salt solutions is increased by the addition of lecithin [236]. The bile salt micelle is said to be swollen by the lecithin until the micellar structure breaks down and lamellar aggregates form in solution the solution is anisotropic. Bile salt-cholesterol-lecithin systems have been studied in detail by Small and coworkers [267-269]. The system sodium cholate-lecithin-water studied by these workers gives three paracrystalline phases I, II, and III shown in Fig. 4.37. Phase I is equivalent to a neat-soap phase, phase II is isotropic and is probably made up of dodecahedrally shaped lecithin micelles and bile salts. Phase III is of middle soap form. The isotropic micellar solution is represented by phase IV. The addition of cholesterol in increasing quantities reduces the extent of the isotropic... 

Figure 4.37 Phase diagrams of sodium cholate (NaC), water (W), lecithin (L). Systems (a) with no added cholesterol (b) with 0.5 % cholesterol (c) with 2 % cholesterol and (d) with 4% cholesterol. In the system without cholesterol the three paracrystalline phases are labelled I, II, and III. Phase I is analogous to neat soap of aqueous soap systems. Phase II is a cubic phase , and phase III is analogous to the middle soap phase of common soap systems. Phase IV is isotropic micellar system. X in diagram (b) represents the composition of normal human gall-bladder bile. From Small et al. [267, 268].

The phase condition for concentrations in the range close to the cmc are found in Fig. 4A. For the lowest soap concentrations, a liquid isotropic alcohol solution separated, when the solubility limit of the alcohol was exceeded. This was changed at concentrations approximately one half the cmc, when a lamellar liquid crystalline phase appeared Instead. After the relatively narrow three-phase region had been transversed, this liquid crystalline phase was the only phase in equilibrium with the aqueous solution. Solubilization of the long chain alcohol Increased at the cmc, as expected. 

Interaction with water, and dispersion into large aggregates. In samples containing appreciable sodium oleate, a turbid dispersion was present. Microscopically, this phase contained isotropic oil droplets, and the phase should represent emulsified fatty acid and monoglyceride droplets stabilized by acid soap. Unfortunately, x-ray diffraction facilities were not available to characterize this phase properly. 

Experimental results (12) showed a transition to a lamellar liquid crystal for 14 added water molecules. Our calculations (to be reported at a later occasion) showed no discontinuity or any other indication of instability of the soap/acid water complex for the subsequent water molecules added in excess of 14. It appears reasonable to assume that the isotropic liquid/liquid crystal transition does not depend on the energy levels of the polar group interactions. The phase transition probably depends on the hydrophobic/hydrophilic volume ratio and estimations according to Israelachvili/Ninham (15) approach may offer a better potential for an understanding. 

The fifth main type occurs in systems in which the soap component is not an association colloid of the paraffin chain type but a salt of a bile acid, with its condensed four-ring skeleton with two or three hydroxyl groups and with one carboxyl group at the end of a branched hydrocarbon chain. Figure 29 shows the phase diagram for the sodium cholate-decanol-water system (9). There is no mesomorphous phase but one extensive continuous area with homogeneous isotropic solutions. The cholate and decanol are mutually soluble in the presence of water, as in the case of the soap and the alcohol in the soap-alcohol systems, but here we have the remarkable phenomenon that water and decanol, which are practically insoluble in one another, become mutually soluble in all proportions in the presence of a certain quantity of a bile acid salt. 

There is plenty of information available in literature on liquid crystalline and isotropic phases of different surfactants, but the study of the solid phases is limited to only soaps [12-19]. The basic reason for the lack of solid phase studies of synthetic surfactants is their limited use in solid form. The low KP of these surfactants limits their use to liquid products. Owing to a relatively high KP, sodium cocoyl isethionate (SCI), alkyl glycerol ether sulfonate (AGES), and sodium cocoyl monoglyceride sulfate (CMOS) form solid phases at room temperature, but there is hardly any information available on these solid phases. 

Different materials such as salts, free fatty acids, polyols, fatty alcohols, fatty esters, and per-fiunery components can influence the formation of liquid crystalline phases. Free fatty acids and fatty alcohols promote the formation of lamellar liquid crystalline phase [26], One can expect solid, isotropic solution, and hexagonal liquid crystalline phases coexisting in normal soaps, but in superfatted soaps, part of the hexagonal liquid crystalline phase is converted to lamellar, which is responsible for product softness during processing. 

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